The invention relates to a stator blade assembly with a plenum inward of the inner blade platform, including a plate with platform impingement cooling apertures and a flow metering aperture to control air flow from the outer shroud through a passage in the leading edge portion and air pressure within the plenum.
The turbine section of a gas turbine engine includes stator blade assemblies or stationary vanes between turbine rotors with rotor blades. The stationary vanes or stator blades are circumferentially arranged in rows with an airfoil profile formed between an inner shroud and an outer shroud that contains the annular hot gas path. Vanes are exposed to hot gas delivered from the combustor and cooling of the stator vanes is extremely important for engine service life. Normally, cooling is provided by bleeding off and ducting a flow of compressed air from the low pressure stage or high pressure stage of the compressor through various passages formed within the stator vanes and exhausting the cooling air into the hot gas path at the trailing edge of the blade.
In one conventional gas turbine engine arrangement, high pressure compressed air is bled from the high pressure plenum surrounding a reverse flow combustor that is adjacent to the first or second row of stationary stator vanes or blades. High pressure compressed air is somewhat higher in temperature than the low stage compressed air. However, due to the proximity of the high pressure plenum around the combustor, it is common to simply duct the hotter high pressure air rather than incur the weight penalty of ducting cooler lower pressure air a longer distance from the low stage compressor area.
Cooling air from the stator blades eventually enters the hot gas path flowing through the turbine section. However little useful work is obtained from the cooling air. Therefore, to achieve high efficiency it is critical that the cooling air be effectively utilized to minimize the amount of cooling air and the penalty imposed on the engine by bleeding compressed air for cooling purposes.
U.S. Pat. No. 5,609,466 to North et al. shows a prior art cooled inner shroud where a portion of the cooling air that is ducted through the stator blades is used to cool the inner shroud. The inner shroud is cooled by impinging cooling air against the inner shroud surface and directing cooling air through passages in the downstream blade platform of the inner shroud to exhaust the cooling air into the gas path. For this purposes a plenum is formed on the underside or inner surface of the blade platform.
As shown in U.S. Pat. No. 5,609,466 to North et al. as well as U.S. Pat. No. 6,089,822 to Fukuno, compressed air is fed through the outer shroud into channels formed within the stator blades. The major portions of the cooling air is ducted through channels in the blade and exits into the hot gas path either at the trailing edge of the blade or partially through effusion apertures to form a cooling curtain around the exterior air foil surface and particularly the leading edge portion of the blade.
However, in order to cool the blade platform such prior art blades include a plenum formed inward of the blade platform to contain compressed air that is ducted through the blade and into the plenum. Compressed cooling air within the plenum is then ducted with a plurality of impingement holes formed in a cover plate to form jets of compressed cooling air directed to the inner surface of the blade platform. Thereafter, the air is ducted through further channels in the down stream portion of the platform to exit into the hot gas path. Optionally, the area around the plenum may be purged with cooling air also ducted through the plenum and out purged openings in the plenum enclosure to purge stagnant hot gases from around the plenum and rotating turbines then to rejoin the hot gas path.
As it is well known to those skilled in the art, the controlling of cooling air and minimization of the amount of cooling air used, is a major factor in the engine efficiency. Leakage of cooling air represents a significant penalty on the engine efficiency. In effect, the less cooling air that is needed the better and significant design effort is expended to optimize the use of cooling air.
A significant disadvantage of prior art devices is the failure to accurately meter the flaw of cooling air that passes through the channels and the blades into the plenum enclosure for impingement cooling of the blade platform area. For example in U.S. Pat. No. 5,609,466 to North et al. the flow of cooling air that eventually enters the plenum may come from various sources at various temperatures and pressures. Air may flow directly through a hole in the inner side of a tubular insert member, or may come from an annular area around the tubular member that has been cooled with air exiting numerous openings in the tubular member to cool the blade interior. Further, since North et al. uses a first tubular insert in the leading edge portion and a second tubular insert in the tubular edge portion, the flow of compressed cooling air that enters the plenum beneath the blade platform may come from four different sources, all of which have different pressures and temperatures as a result of their varying flow path.
The failure of such prior art systems to accurately meter the flow of air that enters the plenum, results in unpredictable performance and excessive leakage from the plenum through axial joints between the stator blades. The complexity involved in delivery of different flows of compressed air to the plenum makes control and predictability extremely difficult. Reliance an experimental results is unsatisfactory since the design of the blade castings has already been committed to by the time experiments can be performed.
U.S. Pat. No. 6,089,822 to Fukuno somewhat alleviates this problem by directing some of the flow from the trailing edge insert directly into the plenum. However, flow from the insert is also mixed with flow that has exited through perforations in the insert and mixing of cooling are of different temperatures and pressures inevitably occurs adding to the unpredictability of the system.
It is an object of the invention to provide highly accurate metering of cooling air delivered to the plenum on the inner surface of the blade platform to accurately and predictably deliver a controlled amount of cooling air thus enabling rational optimization of cooling air use.
It is a further of the invention to provide a simple means by which air for cooling of the blade can be accurately and predictably split between cast cooling passages within the blade itself and the plenum that supplies impingement cooling air for the inside surface of the blade platform as well as purging of adjacent areas.
It is a further object of the invention to accurately meter the flow of cooling air into the plenum on the inner surface of the blade platform by use of a highly accurate manufacturing process.
Further objects of the invention will be apparent from review of the disclosure, drawings and description of the invention below.
The invention provides a stator blade assembly for a gas turbine engine having: an outer shroud with an air supply port in communication with compressed air from a high pressure stage of a compressor of the engine; an inner shroud including a blade platform and a plenum enclosure defining a plenum bounded by an inner surface of the blade platform; and a blade spanning between the outer and inner shrouds.
The blade has a leading edge portion with a passage communicating between the plenum and the air supply port of the outer shroud and an internal blade cooling channel communicating between the passage and apertures adjacent the trailing edge of the blade.
The plenum includes an impingement plate disposed a distance from the inner surface of blade platform to define an impingement cooling chamber within the plenum, and the plate includes impingement cooling apertures to direct cooling jets of air at the inner blade platform. An air flow restriction plate covers the inner end of the passage and controls the pressure and quantity of air delivered to the plenum via a compressed air metering aperture. Preferably the impingement plate and flow restriction plate are manufactured as a one-piece unitary cover plate sealed to the inner surface of blade platform and covering the inner end of the passage.
A vent extends between the impingement cooling chamber and an outer surface of the blade platform venting to the hot gas path of the engine. As well, a purge bore may extend between the plenum and an outer surface of the plenum enclosure to purge adjacent areas and exhaust to the hot gas path of the engine.
In contrast to the unpredictable uncontrolled flow of cooling air in the prior art, the invention provides a very simple means to meter or control the flow of cooling air into the plenum that supplies impingement cooling air to the inner surface of the blade platform.
As a result, the pressure of air within the plenum is controlled as well as the volume of flow through to optimize use of cooling air and minimize leakage losses. A unitary cover plate is sealed on the under side or inner side surface of the blade platform and covers an inner end of the passage which delivers fresh air from the compressor through the blade itself. The plate can be accurately produced to very high tolerance with drilled holes for impingement cooling as well as a drilled hole for metering the compressed air. Casting tolerances are much higher than those achieved through drilling of a simple plate. The flow restriction hole can be accurately produced to high tolerance whereas castings generally have a much larger range of tolerance and therefore introduce higher inaccuracies in controlling the flow air.
The invention therefore capitalizes on the low cost and relatively liberal tolerance requirements of casting processes in forming passages through the blade for the bulk of the cooling air and uses an accurately drilled flow restriction hole in a cover plate to control and meter the proportion of cooling air that is split off into the plenum and used for impingement cooling of the inside surface of the blade platform.
As a result, the amount of cooling air that is directed to the plenum can be accurately controlled, modified, predicted and monitored. Experimental testing may determine the precise optimum flow split between the air delivered to the serpentine channels within the blade and to the impingement cooling plenum on the inside surface of the blade platform. Further since such components are exposed to high heat and airflows, frequently placement and maintenance are required for optimum performance. The use of a drill plate that can be removed and replaced easily significantly reduces the cost and labour involved since accuracy can be maintained by replacement of the plate and air flow adjustment can be accomplished by re-drilling the flow restriction hole if additional flow is required.
Therefore, the invention provides a simple and effective means to accurately control the proportion of cooling air that is divided between cooling channels within the blade itself and delivery to the impingement cooling plenum for impingement cooling of the inside surface of the blade platform. By accurately sizing the opening in the plate for flow restriction, the temperature and pressure of cooling air within the plenum can be accurately controlled. Optionally, in addition to impingement holes in the plate for impingement cooling of the blade platform, air can escape from the plenum through air purged bores extending between the plenum and the outer surface of enclosure to purge hot gases that are trapped between the rotating turbine components and the stationary blade plenum.
Modification of the optimum flow split is extremely simple, merely requiring the resizing of the metering aperture. Further advantages of the invention will be apparent from the following detailed description and accompanying drawings.